I wouldn't suggest you go SLI in the first place. I saw you have a 8600GT which is far from being a powerful card, having 2 wouldn't be great either. I'm quite sure 1 8800GTS 320mb would beat that card with ease. Going SLI is usually a waste of money because it gives small benefits compared to costs of cards, it costs more in mobo and you need a more powerful PSU because of it. It will not do as well as a next generation card or in your case, as a more powerful one.
Just looking at Tom's hardware charts, 1 8800GTS is about twice as powerful as 1 8600GT. You do not get twice the performance by going SLI, so that's where it ends IMHO.